1. Field of the Invention
This invention generally relates to a device for detecting the quantity of remaining developer which is for application in the dry type developing device of an electrophotographic copying machine. More particularly, it relates to an improved dry type developing device which can accurately detect when the developer in the developer tank or developing means of such developing device is below a predetermined quantity.
2. Description of the Prior Art
In electrophotographic copying machines of the dry development type, development is usually effected by the use of powdered developer after an electrostatic latent image is formed on a photosensitive medium. The dry type developing devices include those which adopt various types of means such as magnetic brush, cascade, fur brush, powder cloud, etc. and in any of these, the powdered developer is supplied from a hopper-like developer tank into developing means, as required. Such developer tank may assume various configurations such as; an open-bottomed tank having a concave-convex surfaced rotary roller provided in the bottom opening thereof for causing the powdered developer to be dropped for supply by the rotation of the roller; an open-bottomed tank having a rotary screw roller provided in the bottom thereof for delivering the powdered developer by the axial thrust imparted by the rotation of the screw roller; or an open-bottomed tank having a member provided in the bottom opening thereof for causing the powdered developer to be dropped for supply by the reciprocal movement of the bottom member. In these developer tanks, it is important in producing stable copy images that, when the powdered developer continues to decrease in quantity from full condition to deficient condition, such deficient condition be detected to provide a fresh supply of developer or to stop the machine from operating. The present invention intends to provide a device which can accurately, reliably and simply detect a point of time at which such deficiency of the developer occurs.
In the electrophotographic copying machines of the liquid development type, detection of the quantity of remaining developing liquid could readily and accurately and reliably be accomplished by using a float or like means to detect the level of the liquid, or by using a transparent tank to effect the detection by the varying quantity of light passed through the developing liquid in the tank. However, these detection methods are not available for the developer tanks containing powdered developer therewithin, because such developer tends to deposit on the tank wall to intercept the passage of light therethrough and thus make the detection impossible or because the powdered developer does not aways decrease its quantity uniformly within the tank but rather the quantity of the developer depositing on the tank wall does not decrease during development, thus making it impossible to use a float or the like for the purpose of detecting the remaining quantity. Thus, in the dry type electrophotographic copying machines, there has been developed no device for detecting the quantity or remaining developer, which device is low in cost and accurate, reliable and stable in operation.
Besides liquid developer detectors, there are som devices which utilize variations in impedance between an electrode provided in the supply container and the wall of such container or which utilize variations in resistance of a rotary impeller installed within the supply container, but none of these devices have been put into practice for the purpose of detecting the quantity of remaining developer in electrophotography.